IMPROVEMENTS RELATING TO ROTARY DRIVE MECHANISMS

Abstract: A high-precision drive motor (1) comprises a drive cylinder (2) which has a first cylindrical contact surface (8) and a driven cylinder (3) which has a second cylindrical contact surface (9) of a slightly different circumference to that of the first surface (8). The drive cylinder (2) is mounted by means of a plurality of controllable length struts (4) so as to be displaceable around a central axis (7). The struts (4) are arranged to be driven such that the first surface (8) comes into pressure contact with the second surface (9) at a single location (6) which rotates around the axis (7) when the struts (4) are cyclically driven, thereby causing the driven cylinder (3) to rotate.

Description: IMPROVEMENTS RELATING TO ROTARY DRIVE MECHANISMS Field of the Invention This invention concerns improvements relating to rotary drive mechanisms and more particularly relates to a high-precision drive motor suitable for use in a rotary stage of the mechanical manipulator that is described in O-A-9217313 for example. Background of the Invention In many fields of engineering, there is a growing need to control precise rotation at low speeds, particularly to facilitate computer controlled positioning systems for example. This need is currently satisfied by use of combinations of standard motors and speed reducers of various kinds, the aim being to provide mechanical stiffness at high loads and in a low friction package, the latter requirement being to enable fine incremental movement without starting friction (stiction) . Some speed reducer designs incorporate toothed gears which can carry high loads and can be pre-loaded to reduce backlash, but their mode of operation causes rubbing friction with consequently heavy stiction and wear. Because it is impossible to form the teeth of the gears perfectly and provide optimum meshing with adjacent gears, gear systems cannot deliver a high- precision constant and controllable rotation. Other speed reducer designs involve belts or bands to transmit the power, but a problem with these is to maintain stiffness in an intrinsically flexible system. One example of a geared speed reducer is the so- called harmonic drive as described in US 4 823 638 or US 2 906 143 for example. A harmonic drive incorporates a toothed flexible spine which functions as a speed reducer for a faster rotating elliptical wave generator. The flexible spline is coupled to a toothed non-flexible output spline at two moving tooth engagement zones. Stiffness in this inherently flexible system can be improved by use of special gear-tooth profiles, but the output stiffness and torque are nonetheless limited and in addition there are inherent problems with backlash. These limitations make the harmonic drive unsuitable for many high-precision high-torque applications. Objects and Summary of the Invention The object of the present invention is to overcome or at least substantially reduce the above mentioned problems. According to one aspect of the present invention there is provided an epicyclic motor wherein movement is derived by means of one or more members having controllably adjustable lengths. More particularly, the present invention provides a high-precision drive motor comprising a drive cylinder mounted by means of ' a plurality of controllable-length struts so as to be displaceable about a central axis, said drive cylinder having a first cylindrical contact surface, and a driven cylinder rotatable about said axis, said driven cylinder having a second cylindrical contact surface of a slightly different circumference to that of the first surface and said plurality of struts being arranged to be driven cyclically such that the first surface comes into pressure contact with the second surface at a single location which rotates around said axis thereby causing the driven cylinder to rotate. The drive cylinder is preferably located coaxially with the driven cylinder, and in addition the struts are preferably of equal length and disposed symmetrically about said central axis. The drive cylinder may for example be mounted by means of six struts arranged in pairs to form a triangular or delta-shaped strut array and with this arrangement the drive cylinder may be mounted at the vertices of the delta-shaped strut array in the case where the drive cylinder is inside the driven cylinder, or at the mid- points of the sides of the delta-shaped strut array in the case where the driven cylinder is inside the drive cylinder. Alternatively, the drive cylinder may be mounted by means of more than six struts arranged to form a regular array which can resolve the rotational output force into contraction/extension components at the locations of the struts. Furthermore, the drive or driven cylinders may be extended outside of a plane in which said struts are located to provide for a flexible range of gear ratios. The struts preferably comprise magnetostrictive, electrostrictive or piezoelectric material which is subject to dimensional change in response to magnetic, electric or electromagnetic influences and may for example comprise Terfenol which is a highly magnetostrictive material available from Johnson Matthey Rare Earth Products. An electrical drive source arranged to supply control signals for the cyclic stressing of the struts may, in the case where the struts comprise magnetostrictive material, be operatively coupled to the struts by means of a plurality of windings formed around the respective struts and arranged to be driven individually and preferably in a polyphase manner so as to obtain the requisite rotation of the pressure contact location between the drive and driven cylinders. The struts are preferably arranged to be pre¬ loaded and, furthermore, desirably are so mounted for example by elastomeric means as to accommodate a degree of movement of the ends of the struts during their cyclic expansion and contraction. In order to reduce vibration during operation, the motor preferably further comprises counterbalance means arranged to maintain the centre of gravity of the drive cylinder on or substantially on said central axis during its displacement, and the counterbalance means may advantageously comprise a plurality of hydraulic counterbalances. Alternatively or additionally, the counterbalancing may be achieved by coupling together two or more of said motors arranged for operation out of phase with each other so that the vibrations of the motors cancel. In a further alternative; a further drive cylinder mounted by means of a further plurality of controllable-length struts and displaceable about the central axis, may be provided. The struts would be arranged to exert equal and opposite forces on their respective drive cylinders along a vector passing through a common point on the axis. The contact surfaces of the drive and driven cylinders may advantageously be processed to provide a high degree of friction on a relatively smooth surface, and may for example be coated with a thin film of a hard material having a microcrystalline structure such as diamond or Tungsten Carbide for example. Additional friction between the contact surfaces may be provided by making these surfaces non- planar and generally complementary such that they are capable of being wedged together. These contact surfaces may for example, have wedged or curved cross- sectional profiles. Alternatively, for larger motors the said surfaces could be provided with machined surface textures including low-profile gears. As will be explained in the following with reference to exemplary embodiments, the cyclic energization of the struts can be arranged to achieve an epicyclical, non-rotating displacement of the drive cylinder about the central axis such as to cause the contact between the drive and driven cylinders to rotate. By arranging the contacting surfaces of the two cylinders to have only slightly different diameters so that contact is maintained between the drive and driven cylinders as the contact location rotates, a counter rotation of the driven cylinder is obtained. The cylinders can be manufactured with high precision so as to provide for constancy and uniformity of rotation, and a high torque results from the large reduction ratio which results from the close diameters of the co-operating cylindrical surfaces. The invention may alternatively be stated to provide: a drive unit for providing high-torque low- speed precision rotation, said unit comprising a first cylinder and a second cylinder, said first cylinder being positioned within said second cylinder and having an external circumference slightly smaller than the internal circumference of the second cylinder, and a plurality of controllable-length struts for supporting one of said cylinders, said struts being arranged to be driven in order to displace said one of said cylinders about a central axis such that it comes into cyclic pressure contact with the other of said cylinders thereby causing said other cylinder to be rotated about said axis; a drive comprising a plurality of lengthwise adjustable struts arranged to be driven cyclically so as to exert a mechanical tangential force component on a cylindrical surface so as to cause it to rotate; - a drive comprising first and second cylindrical members mounted one within the other, one rotatable and one not, and wherein means are associated with the fixed cylinder for causing a region of contact between it and the rotatable cylinder to be cyclically displaced so ' as to effect a counter rotation of the rotatable cylinder. The above and further features of the present invention are set forth with particularly in the appended claims and should become clearer from consideration of the following description of exemplary embodiments of the invention given with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 is a cross-sectional view of a magnetostrictive drive motor according to a first embodiment of the invention, wherein an epicyclically displaceable non-rotating drive cylinder is provided within a rotatable driven cylinder; Figure 2 is a schematic fragmentary cross- sectional view of an hydraulic counterbalance which is included in the embodiment of Figure 1; Figure 3 is a cross-sectional view of a magnetostrictive drive motor according to a second embodiment of the invention, wherein a driven cylinder is provided within an epicyclically displaceable non- rotating drive cylinder in an opposite arrangement to that of Figure 1; Figure 4 is an end view of a modification to the magnetostrictive motor of Figure 1 or 3 wherein the drive and driven cylinders are extended outside the plane of a strut array to provide a flexible range of gear ratios; Figure 5 is a schematic cross-sectional view of a magnetostrictive motor according to a third embodiment of the invention wherein the number and the arrangement of the tension/compression members utilized in the drive cylinder is different from that shown in Figures 1, 3 and 4; Figures 6A, 6B and 6C are schematic cross- sectional profiles of the various contact surfaces between the drive and driven cylinders that can be used to improve friction; and Figure 7 is a schematic cross-sectional view of a magnetostrictive drive motor according to a further embodiment of the invention. Detailed Description of the Embodiments Referring now to Figure 1 of the accompanying drawings a magnetostrictive torque motor 1 comprises an epicyclically displaceable, non-rotatable drive cylinder 2 located within a rotatable output or driven cylinder 3 constrained to rotate coaxially with the drive cylinder mounting. Six magnetically extensible and contractible struts 4 are provided within the drive cylinder 2, to cause a contact region 6 between the two cylinders to rotate and thereby exert a mechanical tangential force component on the inner surface 9 of the driven cylinder 3 as will be explained hereafter and cause it to counter-rotate. The rate of rotation of the driven cylinder 3 is the epicyclic rate of displacement of the drive cylinder 2 multiplied by the difference in the circumferences of the contact surfaces.8, 9 of the two cylinders 2, 3 divided by the circumference of the contact surface 9 of the driven cylinder 3. Cyclic operation of the struts 4 causes the non- rotatable drive cylinder 2 to be displaced epicyclically about the axis 7 of its mounting, such that the contact region 6 on its circumference where it bears internally upon the larger diameter driven cylinder 3 is caused to rotate about the epicyclic axis 7. The drive cylinder contact surface 8 has a slightly smaller circumference than the contact surface 9 of the driven cylinder 3, so that if slipping and skidding of the contacting surfaces relative to one another is to be avoided the driven cylinder 3 has to rotate in the opposite direction to the epicyclic movement of the drive cylinder 2. Since the contact surfaces 8, 9 of the two cylinders 2,3 differ only slightly in circumference, there results a large reduction between the rate of epicyclic motion of the drive cylinder 2 and the speed of rotation of the driven cylinder 3. The motor 1 presents significant advantages over the prior art mechanisms discussed hereinbefore, namely it is inherently stiff and has a high-torque output and it presents a high degree of incremental controllability. The stiffness of the motor 1 is a consequence of the hi.gh pressures that can be generated at the interface of the two cylinders 2, 3 and the high torque results from the implicit large reduction ratios. Incremental controllability results from the fact that the motor can have a very low friction of rotation and a high degree of constancy of rotation resulting from the high precision with which the cylinders 2, 3 can be manufactured. Other advantages of the motor 1 include compactness and light weight which result from the simplicity of its design and its minimal moving parts. Referring now to Figure 1 in more detail, six struts 4 are used in the embodiment in order to ensure stable operation of the motor 1 with all forces triangulated. The struts 4 are arranged in three pairs to form a triangular or delta strut array. At one end 10 the struts 4 each connect to a respective one of three fixed mountings and at their other ends 12 the struts 4 each connect to respective mountings formed inside the drive cylinder 2. As will be described in more detail hereafter, the mounting of the struts 4 is such as to place them under a compressive pre-load and such as to permit a degree of pivotal movement at the respective strut ends. The struts 4 in this embodiment are made of a magnetostrictive material, such as Terfenol for example, which demonstrates a significant length variation when subjected to the influence of a strong magnetic field. Terfenol can provide dimensional variations of up to 0.3% and this could for example result in a displacement of 0.3mm for a 100mm length. Strain forces generated by a Terfenol strut can be very large and can for example amount to 1000 kgs in a 10mm diameter strut. The magnetisation required to produce a variation in the lengths of the struts 4 is achieved by provision of a solenoid winding 15 around each strut 4. For greatest effect the windings 15 extend over as much of the active length of the struts 4 as possible. The magnetostrictive length variation depends only on the magnitude of the excitation currents within the solenoid windings 15 and is unidirectionally independent of the sign of the applied magnetomotive force. The cyclical length variations of the struts 4 are controlled so that the drive cylinder 2 is epicyclically displaced as a whole rather than distorted and a computer control system (not shown) maintains this operating pattern by controlling the distribution of currents to the solenoid windings 15. The method of controlling the cyclic stressing of the struts 4 is described in more detail hereinafter. The magnetostrictive material employed operates more effectively when pre-loaded and for this purpose three wedges 16 are provided at the vertices 12 of the delta strut array which cause the struts 4 to be compressively pre-loaded when assembled into the array. Elastomeric pads 17 are provided in the wedges 16 and in the mountings at the other ends of the struts to allow for the small amount of movement of the ends of the struts 4 relative to the wedges 16 that will be experienced during the cyclic expansion and contraction of the struts. Alternatively, each elastomeric pad 17 could be replaced by a simple rod and socket joint at the end of each strut 4. The socket would be a cylinder which allows rotation of the rod within the cylinder in a plane perpendicular to the axis of the cylinder. The benefits of utilising a mechanical joint, for example, improved wear characteristics, could thus advantageously be obtained. To ensure that the two cylinders 2, 3 do not slip relative to each other the contact surfaces 8,9 of the two cylinders 2, 3 are flame coated with Tungsten Carbide. This has a icrocrystalline surface structure which acts as miniature gear teeth in resisting slip, whilst being hard and tough enough to resist the mating and drive drag pressures. Many other surface preparations can be utilized as long as they fulfil the condition of providing high friction on a relatively smooth surface. As the motor is scaled up or otherwise varied to increase the size of the epicyclic displacement, other machined surface textures could possibly be utilised. For example, axially patterned machine texturing (such as low profile gears) between the drive and driven cylinders 2, 3 can improve friction and provide indexable reference positions. A radial pattern may also increase the friction between the surfaces 8, 9. Now referring to Figures 6A to 6C, there are shown various cross-sectional profiles of the contact surfaces 8, 9 of the drive and driven cylinders 2, 3, that can be incorporated into the design of the motor. More particularly, Figure 6A shows standard flat contact surfaces 8, 9 coated with a layer of Tungsten Carbide 50 which acts as miniature gear teeth in resisting slip as described previously. Figures 6B and 6C respectively show peaked and curved generally complementary male and female contact surfaces 8, 9 which improve the contact between the drive and driven cylinders 2, 3, whilst also providing a means for maintaining a constant planar positional relationship between the drive and driven cylinders 2, 3. There are two significant advantages in using the non-planar complementary contact surfaces 8, 9, shown in Figures 6B and 6C. Firstly, their use gives an increased area of contact between the drive and driven cylinders 2, 3 which increases the friction therebetween. Secondly, their use causes an increase in the net pressure between the contact surfaces because of the inherent "wedging effect" which also increases the friction. The additional frictional force generated by this "wedging effect" may be sufficient to remove the need to use the flame coated Tungsten Carbide layer 50. Alternatively, the non- planar contact surfaces 8, 9 could be used in conjunction with the Tungsten Carbide layer 50 or other coatings to provide different degrees of friction. The epicyclic motion of the drive cylinder 2 would normally generate a vibration caused by its mass rotating off-centre. This could be balanced by coupling the described motor to at least one further such motor with the coupled motors arranged to run suitably out of phase such that the centre of gravity of the combined motors lies permanently on the axis 7, with the speed at which each motor is rotated being controlled so that there is no overall turning moment acting on the axis 7. However, in the embodiments described herein, the tendency of the motor to give rise to vibration is compensated by means of hydraulic counterbalances 14. In Figure 1, three such hydraulic counterbalances 14 are provided at the mid points of the three limbs of the delta strut array, the three counterbalances all being identical and being mounted in respective bores formed in flanged portions 29 provided within the drive cylinder 2 so as to bear against the radially outer ends of the mountings of the ends 10 of the struts 4. Figure 2 provides an enlarged showing of one of the hydraulic counterbalances and it will be seen that it consists of a member 30 secured in the bore 31 in flange portion 29 and capturing in the bore a counterweight 32 and a set of disc springs 33. The counterweight has a relatively large head 34 which makes an easy sliding fit within the bore 31 and has a relatively small neck 35 which makes a close sliding fit within the member 30 which has an upstanding collar portion 36. A silicone rubber seal 37 is mounted in an undercut part of the head 34 and seals the head 34 of the counterweight against the outer surface of the collar portion 36. On the other side of the member 30, a chamber 38 is defined by a disc 39 which is sealed to the member 30 by an elastomer seal 40 and the chamber 38 is filled with a hydraulic fluid. As shown, the member 30 is so shaped that the chamber 38 has a large area on its radially inner side where the disc 39 bears against the mounting 13 and has only a relatively small area on its opposite side where the end face of the neck 35 of the counterweight. During epicyclic motion of the drive cylinder 2 the disc 39 is pressed against the shaft mounting 13 which causes the elastomer seal 40 to compress so that the hydraulic fluid in the chamber 38 moves the counterweight 32 by an amount which is proportionately greater in the ratio of the surface areas of the disc 39 and the end face of the counterweight neck portion 35. The seal 40 also accommodates sideways movements. The counterweight 32 is urged in the opposite direction by the set of disc springs 33 which have a sufficient force to maintain a positive pressure in the hydraulic fluid within chamber 38. This means that the maximum return pressure is not limited to 1 bar, and ensures a much higher resonant frequency for the counterbalance assembly than would be experienced during motor operation. As will readily be understood, an epicyclic displacement of the mounting 13 may for example give rise to an off-centre mass of lOOgms, so that if the weight of the counterweight 32 (including displaced fluid) is lOgms and the ratio between the cross-sectional areas of the large and small sides of the hydraulic chamber 38 is 10:1, then the large mass of the drive cylinder 2 may displace by 0.1mm and the counterbalance will displace by 1.0mm. Displacing the counterweight 32 by the displacement of the cylinder multiplied by the area ratio in the hydraulic cylinder 38 retains a constant centre of gravity. Electrical excitation of the solenoid windings 15 is preferably performed in a polyphase manner. In a simple case, this excitation may take the form of sinusoids of current or some function of sinusoids of current offset with respect to a mid-point value and displaced in phase each with respect to the others according to the number of struts in the machine and the geometry of their deployment. Typically: I(t)=Ip(l+sin(wt+P) ) /2 where P is an inter-strut phase displacement. The magnetisation offset may be provided either by providing a standing bias current in the solenoid windings 15 or by means of permanent magnets. Any suitable repetitive phase-displaced time function (table or real time derived) may be utilized. A suitable form will be one which optimises mechanical parameters such as smoothness of evolved torque, velocity or position, or the peak torque or velocity. The electrical drive for the magnetostrictive torque motor 1 may be effected under one or more of several control strategies; for example open or closed loop position. In both cases excitation may be simply by open loop volts or closed loop current. In a typical situation, open loop position and closed loop current strategies may be used in conjunction with polyphase pulse-width modulated electrical drive signals. The solenoid windings 15 in this case may be driven individually or in a pair of star configurations as a zero sequence component is required to offset the periodic excitation. If the connection to the windings 15 is made in a star configuration with access provided to the centre point, an option is available for injecting DC current to offset the strut lengths for setup bias, temperature compensation or other purposes. As an alternative, a closed loop current method may be used alone with the transduction of winding currents and feedback to a suitable power electronic controller to allow precise definition of the magnetisation of the struts according to a predefined pattern, or may be combined with the closed loop position method to give a fully controlled mode of operation. In this case, the output position of the driven cylinder 3 may be sensed by means of a rotary position transducer, for example a resolver or encoder and this position measurement may be used in a closed loop context to ensure that the current excitation is provided to cause the desired rotational translation. In Figure 3 a second embodiment of the invention is shown where the motor 1 is turned inside out as compared with the first embodiment, i.e. the driven cylinder 3 is positioned within the drive cylinder 2. In this embodiment, the vertices 12 of the struts 4 are fixed on a stable cylinder 18 and the bases 10 of the struts 4 are connected to the drive cylinder 2. Cyclic stressing of the struts 4 causes displacement of said bases 10 and hence of the drive cylinder 2. This second embodiment arrangement provides a smaller gear ratio (higher output speed) than can reasonably be provided by the first embodiment. Even though the displacement remains largely the same in both embodiments, the proportional effect on the smaller drive and driven cylinders 2,3 increases the degree of rotation per movement cycle and hence the potential output speed of the cylinder 3. The same reference numerals are used in Figure 3 as were used to designate like parts in Figure 1. Apart from the differences between the arrangements as outlined above, the Figure 3 arrangement is the same as the Figure 1 arrangement and accordingly no further description thereof will be provided herein. Referring now to Figure 4, the first and second embodiments of the invention can be modified to provide a flexible range of gear ratios. The gear ratio is determined by the ratio of the circumferences of the contact surfaces 8,9 of the drive and driven cylinders 2,3. The modification of Figure 4 extends the drive and driven cylinders 2,3 outside the plane of the strut array and hence any geometrical constraints on the selection of the desired ratio are removed. The advantage of using the above mentioned modification is that it allows motor sizes to be standardized without limitation to a set gear ratio. Referring now to Figure 5, the number of struts used within the motor 1 can be greater than six and the arrangement of the struts 4 is not limited to a delta-shaped array, and could be in the form of a polygon or star-shaped structure. Also, the struts 4 need not be of equal length nor need they be disposed symmetrically about the central axis 7. However, the location, number and length of struts used in a particular design of motor 1 are governed by the ability to resolve the rotational forces between the drive and driven cylinders into compression and expansion components at the locations of the struts. In the embodiment of Figure ' 5, sixteen equal length struts 4 are disposed symmetrically about the central axis 7 in a sixteen-sided polygon. This increase in the number of struts 4 incorporated in the motor 1 may have the benefit of improving its smoothness of rotation and displacement but possibly has the disadvantage that it may increase the weight of the motor 1 and make the sequential controlling of the strut movements more complicated. As mentioned previously, it is not necessary to use counterbalances 14 to compensate for vibration caused by the epicyclic motion of the drive cylinder 2. Rather, by coupling a further motor to the described motor and running the motors suitably out of phase, unwanted vibrations can be compensated. However, this is not an ideal solution even though the combined centre of gravity of the two motors lies on the common motor axis 7, as there may be a moving mass couple acting on the plane of the axis which may cause further vibrations. This moment is caused by the necessary offset of the two centres of motion from each other along that axis. Referring now to Figure 7, a further improved embodiment of the invention is shown which incorporates two drive cylinders 2 running suitably out of phase and which cancels any further vibrations that may be caused by the offset of the moving masses. In general, this embodiment incorporates two drive cylinders which are similar to those of the first embodiment and so only the differences between the embodiments will be discussed hereinafter. The motor of the improved embodiment is totally balanced by each of the drive cylinders 2 being axially offset from a new centre of epicyclic thrust, and by this new motor centre 51 being shared by both cylinders 2. Thus, the arrangement of struts 4 extends out of the plane of each drive cylinder 2 to couple to the shaft mounting 13, and force exerted by any strut 4 of either drive cylinder 2 always acts through the motor centre 51. In this embodiment, a polygon or star-shaped array of struts 4 is used to cause the epicyclic motion of the drive cylinders, and this may be provided as the arrangement of extendible legs described in British patent number GB 2 269 552. As can be seen in Figure 7, balanced operation of the motor is maintained by simultaneously exerting equal and opposite forces on the respective drive cylinders to cause their epicyclic displacement, which rotates the output cylinder 3 but cancels any moving mass couple. To further ensure that there is no tendency for the epicyclic drive cylinders 2 to generate axial forces as a result of the axial component of strut forces, the inside surface of the output cylinder 3 has a conical profile such that the epicyclic thrust of the drive cylinders 2 always acts perpendicularly to the surface 9. The epicyclic thrust of each drive cylinder 2 is generated by cyclically stressing the struts 4 as has been described in the first embodiment. However, the contact surface of each drive cylinder has a conical profile which is adapted to match that of the driven cylinder 3. If means are provided for moving the driven cylinder 3 axially with respect to the motor centre 50, and the strut stroke is commensurately increased, then the gearing ratio between the drive and driven cylinders 2, 3 will change, which allows the output speed of the driven cylinder 3 to be controllably varied. This is highly advantageous as it combines a motor and gearbox into a single device. In all the embodiments described above, the drive cylinder 2 is located coaxially with the driven cylinder 3 when the drive cylinder 2 is not being displaced. In this position the drive cylinder 2 at no point makes contact with the driven cylinder 3 and so the driven cylinder 3 is free to rotate. The embodiments could if desired be modified to provide a braking force on the driven cylinder 3 by offsetting the axis of the drive cylinder 2 such that the drive cylinder 2 is always in pressure contact with the output cylinder 3. As a consequence, non-symmetrical cyclic displacement of the drive cylinder 2 would be required to operate the motor 1 and this may require a non-symmetrical array of non-uniform length struts 4. The above embodiments all incorporate magnetostrictive struts 4 which provide the necessary displacements to drive the motor 1. However, other methods of providing controlled displacement of the drive cylinder 2 can be employed in the same motor concept. One example of this is a hydraulic motor wherein the magnetostrictive struts 4 are replaced by hydraulic elements such as metal hydraulic bellows for example. The bellows could for example be arranged in a star array and a central rotating valve set could be provided which synchronises the pressure and exhaust cycles of the bellows. This type of motor can only be realised because the expansions/contractions required for epicyclic motion would be relatively small. The resulting high-torque motor would be seal free and very controllable. Having thus described the invention by reference to the above embodiments, it is to be appreciated that the described embodiments are exemplary only and are susceptible to modification and variation without departure from the spirit and scope of the invention as set forth in the appended claims.

Claims: CLAIMS : 1. An epicyclic motor wherein movement is derived by means of one or more members having controllably adjustable lengths. 2. A high-precision drive motor comprising: a drive cylinder mounted by means of a plurality of controllable-length struts so as to be displaceable about a central axis, said drive cylinder having a first cylindrical contact surface; and a driven cylinder rotatable about said axis, said driven cylinder having a second cylindrical contact surface of a slightly different circumference to that of the first surface; said plurality of struts being arranged to be driven such that the first surface comes into pressure contact with the second surface at a single location which rotates around said axis when the struts are cyclically driven, thereby causing the driven cylinder to rotate. 3. A motor according to claim 2, wherein said struts comprise magnetostrictive, electrostrictive or piezoelectric material. 4. A motor according to claim 3, wherein said struts comprise Terfenol. 5. A motor according to any of claims 2 to 4, further comprising an electrical drive source, said source being arranged to supply control signals for the cyclic stressing of the struts. 6. A motor according to claim 5 wherein the struts comprise magnetostrictive material, the motor further comprising a plurality of solenoid windings each winding being around a respective strut and being responsive to said electrical drive source. f 7. A motor according to any of claims 2 to 6, wherein the struts are arranged to be driven individually and in a polyphase manner. 8. A motor according to any of claims 2 to 7, further comprising stressing means connected to ends of the struts, said stressing means being arranged to pre-load said struts. 9. A motor according to any of claims 2 to 8, further comprising elastomeric means located at the ends of each strut, said elastomeric means being flexible enough to permit pivoting movement of the struts during their cyclic expansion and contraction periods. 10. A motor according to any of claims 2 to 8, further comprising a mechanical joint located at the ends of each strut, said joints being arranged to permit movement at the ends of the struts during their cyclic expansion and contraction periods. 11. A motor according to any of claims 2 to 10, wherein said first and second surfaces are processed to provide a high degree of friction on a relatively λsmooth surface. 12. A motor according to claim 11, wherein said first and second surfaces are coated with a material having a macrocrystalline structure. 13. A motor according to claim 12, wherein said coating material comprises Tungsten Carbide. 14. A motor according to any of claims 11 to 13, wherein said first and second surfaces are non-planar complementary surfaces, which are capable of being wedged together to provide additional friction between said surfaces. 15. A motor according to claim 14, wherein said surfaces have a wedged or curved cross-sectional profile. 16. A motor according to claim 11, wherein said first and second surfaces are provided with low-profile gears. 17. A motor according to any of claims 2 to 16, further comprising counterbalance means arranged to maintain the centre of gravity of the drive cylinder on said central axis during its displacement. 18. A motor according to claim 17, wherein said counterbalance means comprises a plurality of hydraulic counterbalances. 19. A motor according to claim 17 or 18, wherein said counterbalance means comprises at least one further said motor arranged for operation out of phase with the first-mentioned motor. 20. A motor according to any of claims 2 to 16, which comprises a further drive cylinder mounted by means of a further plurality of controllable-length struts so as to be displaceable about said central axis, wherein said struts are arranged to exert equal and opposite forces on their respective drive cylinders along a vector passing through a common point on said central axis. 21. A motor according to claim 20, wherein said plurality and further plurality of struts extend out of the plane of the drive and further drive cylinders respectively. 22. A motor according to claim 20 or 21, wherein the contact surface of the driven cylinder is adapted to resist any axial forces generated by said struts. 23. A motor according to claim 22, wherein said surface comprises a conical surface. 24. A motor according to any of claims 20 to 23, further comprising means for moving said driven cylinder with respect to said common point or for moving said common point with respect to said driven cylinder, such as to effect a change in gearing between said drive cylinder and said driven cylinder. 25. A motor according to any of claims 2 to 24, wherein said drive cylinder is located coaxially with said driven cylinder. 26. A motor according to any of claims 2 to 25, wherein said struts are of equal lengths and are disposed symmetrically about said central axis. 27. A motor according to any of claims 2 to 26, wherein said drive cylinder is mounted by means of six struts arranged to form a delta-shaped strut array. 28. A motor according to claim 27, wherein said drive cylinder is mounted at the ends of said struts which form the vertices of the delta-shaped strut array. 29. A motor according to claim 27, wherein said drive cylinder is mounted at the ends of said struts which form the mid-points of the sides of the delta-shaped strut array. 30. a motor according to any of claims 2 to 26, wherein said drive cylinder is mounted by means of more than six struts arranged to form a regular array which can resolve the rotational output force into contraction/extension components at the locations of the struts. 31. A motor according to any of claims 2 to 28 or 30, wherein said drive cylinder is mounted within said driven cylinder. 32. A motor according to any of claims 2 to 27 or 29 to 29, wherein said driven cylinder is mounted within said drive cylinder. 33. A motor according to any of claims 2 to 32, wherein said drive and driven cylinders are extended outside a plane in which said struts are located to provide a flexible range of gear ratios. 34. A motor according to claim 2, wherein said struts comprise metal hydraulic bellows. 35. A motor according to claim 34, wherein said metal hydraulic bellows are driven by a rotating valve set which is arranged to synchronise the pressure and exhaust cycles of the bellows. 36. A drive unit for providing high-torque low-speed precision rotation, said unit comprising: a first cylinder and a second cylinder, said first cylinder being positioned within said second cylinder and having an external circumference slightly smaller than the internal circumference of the second cylinder; and a plurality of controllable-length struts for supporting one of said cylinders, said struts being arranged to be driven in order to displace said one of said cylinders about a central axis such that it comes into cyclic pressure contact with the other of said cylinders thereby causing said other cylinder to be rotated about said axis. 37. A drive comprising a plurality of lengthwise adjustable struts arranged to be driven cyclically so as to exert a mechanical tangential force component on a cylindrical surface so as to cause it to rotate. 38. A drive comprising first and second cylindrical members mounted one within the other, one rotatable and one not, and wherein means are associated with the fixed cylinder for causing a region of contact between it and the rotatable cylinder to be cyclically displaced so as to effect a counter rotation of the rotatable cylinder.

Inventors: BAILEY RALPH PETER STEVEN, BAILEY, RALPH, PETER, STEVEN
Applicants: BAILEY RALPH PETER STEVEN, GEODETIC MACHINES LTD, BAILEY, RALPH, PETER, STEVEN, GEODETIC MACHINES LIMITED
Application year: 1994
Main classification: H02N2
IPC codes: H01L41, H02N2